The production of ethylene requires a number of process steps through which any of a variety of hydrocarbon feeds can be refined to generate various products including ethylene. The predominate process for producing ethylene is steam cracking. According to this process, hydrocarbon feed is heated in cracking furnaces and in the presence of steam to high temperatures. It is well known in the industry that shorter residence times within the furnaces results in a desirable selectivity to ethylene.
As such, once the desired conversion of feed has been achieved, the process gas must be rapidly cooled, or quenched, to minimize undesirable continuing reactions that are known to reduce selectivity to ethylene. The vast majority of ethylene furnaces currently in use employ so-called “transfer-line-exchangers” (TLEs), also referred to as “quench exchangers”, for this purpose. These devices are heat exchangers that rapidly cool the process gas by generating steam. The resulting steam is typically generated at high pressures (e.g. 600 to 2000 psig; 4150 to 13800 kpag).
Many of the TLEs in service employ a double pipe or double tube construction with the high temperature cracking furnace effluent introduced into the interior pipe, with a cooling medium such as water being introduced into the annular space between the two tubes. Double pipe exchangers may be configured as bundles or as so-called “linear” units. The advantage of the linear type units is that the adiabatic time between the furnace outlet and the cooling tube inlet can be minimized to allow an enhanced ethylene selectivity. Linear units also benefit from the lack of a tubesheet area which would otherwise be exposed to the hot process gas and are thus subject to various mechanical and erosion concerns. Further, in linear units, the process flow is more evenly distributed among the cooling tubes.
In order to achieve best selectivity to ethylene, it is necessary to minimize both the residence time (“fired time”) and the adiabatic time (“unfired residence time”) within an ethylene furnace. The unfired residence time refers to the amount of time required for the process effluent to pass from the fired zone of the furnace to the entrance of the TLE. One set of existing solutions which have been developed to minimize adiabatic time are typically called “close-coupled” type quench exchangers. According to this design, the quench exchanger tubes are connected directly to the furnace effluent tubes without intermediate manifolding.
Examples of this type of exchanger can be found in FIGS. 15, 16 and 17 of Herrmann and Burghardt, “Latest Developments in Transfer Line Exchanger Design for Ethylene Plants”, prepared for the presentation at AlChE Spring National Meeting, Atlanta, Ga., April 1994. Another close-coupled design is presented in U.S. Pat. No. 4,457,364, which discloses a “Close-Coupled Transfer Line Heat Exchanger Unit.” According to this design, “close-coupling” of the quench exchanger is achieved by using a dividing fitting which connects a radiant outlet tube to two or more quench exchanger tubes using a streamlined fitting. Using this arrangement, the quench exchanger tubes have a smaller inside diameter than the radiant tubes which feed them. Although this arrangement does achieve a low adiabatic residence time and thus has high selectivity to ethylene, it has presented problems in practical operation.
In particular, coke segments that have formed on the inner surface of the radiant tubes, when spalled off the tubes, have proven to not always be able to pass through the smaller diameter quench exchanger tubes. As such, furnaces so equipped must periodically be shut down to remove coke blockages from the quench exchanger inlet upstream of the heat exchanger tubes. As a result, current “close-coupled” quench exchanger designs require the quench exchanger tubes to be larger in diameter than the radiant coil outlet tube. Further, it is preferred to have no dividing fittings between the radiant outlet tube and the quench exchanger tube as in the design of U.S. Pat. No. 4,457,364 because these fittings can also create similar blockage problems.
In single pass radiant coil implementations, such as that shown in FIG. 15 of Herrmann, et al., it is possible to complete all the quench exchanger steam generation in a single pass. However, if two radiant tubes are combined into a single, larger diameter quench exchanger tube (as is geometrically advantageous and which eliminates the blockage problems of the U.S. Pat. No. 4,457,364 design), the quench exchanger length may approach or exceed the limits of commercial fabrication and shipping capabilities which are currently at approximately 60 linear feet (18.3 linear meters).
If a U-tube radiant coil is used, the flow rate per tube and the tube diameter increases and it is therefore not always possible to complete the desired steam generation in a single pass. The Herrmann reference presents two solutions in FIGS. 16 and 17, respectively. In the FIG. 16 embodiment, a two pass quench exchanger is used. In FIG. 17, a single pass quench exchanger is close coupled to the furnace coil and the effluent tubes of the single pass exchanger are manifolded together. Steam generation is completed in a circular TLE. Since the manifolding is performed after the effluent is quenched, there is no loss of selectivity to ethylene.
A similar approach to that shown in FIG. 17 may be undertaken using serpentine coils with 4 to 6 radiant tubes per pass. Such tubes generally have inside diameters in the range of 3 to 4 inches (76 to 100 mm). One drawback of this approach is that the close coupled exchanger must be able to cool the furnace effluent to approximately 1100° F. (590° C.) after the first pass to ensure that no reaction occurs in the higher residence time manifolding required upstream of the circular quench exchanger. As a result, this has effectively prevented the use of single pass, close coupled quench exchangers which include ethylene furnace coils having inside diameters of greater than about five inches (125 mm).
Ethylene furnaces are typically used for the production of a wide variety of products. This includes hydrogen at the light end to steam-cracked tar at the heavy end. As a general matter, the heavier the feedstock, the greater the yield of steam-cracked tar. In naphtha crackers, the effluent composition contains a tar content that is high enough that the heaviest components will commence condensing if cooled to approximately 600° F. (315° C.). As feed stocks get heavier, the tar yield rises and the temperature at which condensation commences also rises. Should condensation of the effluent occur in the quench exchanger, heat transfer is substantially impeded and a sharp increase in effluent outlet temperature results.
Since quench exchangers cool the effluent by generating steam at approximately 2000 psig (13,800 kpag) or less, the quench exchanger wall is generally at approximately 635° F. (335° C.) or less. It is therefore very important to prevent areas of low velocity or recirculation eddies in the quench exchanger tubes. If such areas exist, the effluent can be cooled to at or below its dew point and quench exchanger fouling can result.